The present invention relates to a process for drying moist sludge and, more particularly, to an improved method and apparatus for rapidly and efficiently drying moist municipal sludge.
The safe and effective disposal of industrial process sludge and municipal sewage sludge has become increasingly more difficult and costly. The applicable standards regulating the permissible pollutant contents, i.e. heavy metals and pathogens, have become more restrictive. Simultaneously, available landfill space has diminished and standards governing other properties of municipal sludge suitable for landfill, such as moisture content, have also become more restrictive.
Federal standards for the use or disposal of sewage sludge, promulgated by the U.S. Environmental Protection Agency, detail the requirements for land application and surface disposal of municipal sewage sludge. Land application of sewage sludge for agronomic purposes, such as agricultural lands and parks, requires rigid compliance with limits on pollutants, pathogens, and water content. The treated sludge must also meet so called vector attraction reduction requirements to minimize the potential for the spread of disease by flies, rodents, birds and the like.
Municipal sewage sludge can be treated by heat drying to meet the required standards for direct land application. If the sludge is dried by hot gas treatment, using either direct or indirect contact, the moisture content of the sludge must be reduced to 10% or less with the temperature of the gas in contact with the sludge maintained at least at 80.degree. C.
U.S. Pat. 4,827,853 describes a system for drying sludge, particularly metal hydroxide sludge, utilizing indirect hot air drying by heat transfer through metal surfaces which support the sludge. In particular, the wet sludge is pressed into pockets formed by the links of a metal conveyor chain belt, the chain is conveyed over a flat metal surface, and the metal surface is heated from below causing the heat to be conducted to the sludge via the metal conveyor chain links. Though effective for its intended application, which is to dry the hazardous metal sludge slowly and at low enough temperatures to prevent volatilization of the heavy metals, the process is too slow and inefficient for rapid and cost effective drying of municipal sludge.